1. Field of the Invention
The present invention relates to a method of controlling Forward Error Correction (FEC) used in an Ethernet based Passive Optical Network (EPON) system.
2. Description of the Related Art
Referring to FIG. 1, an EPON system performs the role of point-to-multipoint Ethernet link in order to allow communications between an Optical Line Termination (OLT) 101 and one of a plurality of Optical Network Units (ONU) 102-1, . . . , 102-n and between one of the ONUs 102-1, . . . , 102-n and another one of the ONUs 102-1, . . . 102-n in a PON distribution network. The ONUs 102-1, . . . , 102-n are connected to OLT 101 through a 1×n splitter.
Transmission from one of ONUs 102-1, . . . , 102-n to OLT 101 is referred to as an upstream transmission, and transmission from OLT 101 to one of ONUs 102-1, . . . , 102-n is referred to as a downstream transmission. In a PON, an Ethernet frame of a downstream transmission physically has transmission characteristics of broadcast and is received by all of the ONUs 102-1, . . . , 102-n.
However, an Ethernet frame of an upstream transmission is incapable of being transmitted from one of ONUs 102-1, . . . , 102-n to another one of ONUs 102-1, . . . , 102-n.
Due to these PON characteristics, a MultiPoint Control Protocol (MPCP) function that controls a frame transmission time of each one of ONUs 102-1, . . . , 102-n is used to prevent collision between frames when a plurality of ONUs 102-1, . . . , 102-n transmit Ethernet frames upstream to OLT 101. OLT 101 determines an upstream transmission time available for each of ONUs 102-1, . . . , 102-n and informs ONUs 102-1, . . . , 102-n of the upstream transmission times, ONUs 102-1, . . . , 102-n transmits Ethernet frames upstream to OLT 101 during the designated transmission times determined by OLT 101. Since OLT 101 should know if there is an Ethernet frame to be transmitted upstream from one of ONUs 102-1, . . . , 102-n to OLT 101 in order to determine a transmission time, ONUs 102-1, . . . , 102-n notify OLT 101 in advance whether there is an Ethernet frame to be transmitted upstream. As such, information for performing upstream TDMA is delivered between OLT 101 and ONUs 102-1, . . . , 102-n. In this regard, a Media Access Control (MAC) layer generates and terminates a MPCP MAC control frame having the information for performing the upstream TDMA.
It should be possible for a new ONU to participate in a PON link through a plug & play method. When a new ONU connects to a PON link, variables relating to a PON matching between OLT 101 and the new ONU are exchanged within a prescribed time, automatically resulting in a normal operation state. The operation does not involve a negotiation between OLT 101 and the new ONU. A unilateral exchange of operation variables of OLT 101 and the new ONU results in a normal operation. A MAC control frame is used to exchange relating variables.
To allow the new ONU to participate in (or register) a PON link is referred to as discovery, which follows a procedure shown in FIG. 5. OLT 101 generates a broadcast gate frame in which a broadcast LLID and a broadcast MAC address are designated and, designates a time slot for one of ONUs 102-1, . . . , 102-n to upstream access.
An unregistered ONU receives the frame and acknowledges a time slot to request registration to OLT 101. The time slot is referred to as a discovery window. A plurality of unregistered ONUs can request registration simultaneously during the discovery window, and thus registration requests may collide with each other. When OLT 101 generates a gate designating discovery window, OLT 101 transmits a gate message including capability variables such as operation variables so that the new ONU requesting registration knows a receipt performance of OLT 101. When the new ONU requests registration to OLT 101, the new ONU transmits a message including capability variables such as operation variables.
When request for registration of the new ONU, i.e., a register request frame, is transmitted without collision, OLT 101 establishes a logical link with the new ONU and operation variables relating to the link to OLT 101, and transmits a Logical Link Identification (LLID) to the new ONU. A register MAC control frame is used to transmit a completed register of the new ONU, the LLID, and echo information of operation variables of the new ONU, etc. to the new ONU.
Since the new ONU does not know an LLID that is assigned to the new ONU, a register MAC control frame is transmitted as a broadcast LLID. However, a Destination Address (DA) is designated as a MAC address that is transmitted to OLT 101 when the new ONU makes a register request. The new ONU receives a register frame and acknowledges a normal registration. Thus, when OLT 101 transmits a gate frame, the new ONU transmits a register acknowledge frame using a time slot designated by the gate frame to inform OLT 101 that the normal registration is acknowledged so that both OLT 101 and the new ONU are in a normal operating state.
When a request for registration of the new ONU is erroneously transmitted to OLT 101 due to collision or a link error during a discovery window time, OLT 101 does not generate a register frame, or echo information of operation variables in a register frame of the new ONU is not consistent with the echo information initially transmitted by the new ONU. In this case, it is determined that the new ONU has failed to register. Hence, the new ONU attempts to register again after a proper back-off operation.
Each of ONUs 102-1, . . . , 102-n locates at a predetermined distance from OLT 101 in a PON distribution network. When OLT 101 assigns an upstream transmission time to one of ONUs 102-1, . . . , 102-n using a TDMA method, it is necessary for the upstream transmission time to be compensated for as much as a Round Trip Time (RTT). Through the compensation, there is no possibility of collision of upstream signals when OLT 101 receives the upstream signals.
Therefore, RTT should be measured to determine the compensation required, which is referred to as ranging.
As shown in FIG. 2, MAC/PHY (physical) layers of an EPON system comprise a MAC sublayer, a Reconciliation Sublayer (RS), a Physical Coding Sublayer (PCS), a FEC sublayer, a Physical Media Attachment (PMA) sublayer, and a Physical Media Dependent (PMD) sublayer. A FEC sublayer comprises a transmitter, a receiver, and a synchronous block. A transmitter is illustrated in FIG. 3A. A packet boundary detector 304 performs a boundary identification of an input 8B10B encoded frame from a PCS sublayer using a Ten Bit Interface (TBI). An 8B10B decoder 301 8B10B -decodes the frame into a data frame in byte units, and divides it into blocks in a unit of 239 bytes. A FEC encoder 302 Reed-Solomon-<239, 255, 8>-code-encodes data blocks divided in 239-bytes unit and generates parities composed of 16 bytes for the data blocks, respectively. A parity octets buffer 303 stores the generated parities.
A selector 305 selects one of the outputs of the 8B10B decoder 301 and the output of the FEC encoder 302 and transmits it to an 8B10B encoder 306. When the 8B10B encoder 306 8B10B-encodes the Reed-Solomon encoded data and the generated parities for output, frame identifiers are used to generate a FEC frame, which is transmitted to a PMA sublayer using the TBI, as shown in FIG. 4. The identifiers used to generate the FEC frame are S_FEC indicating the beginning of the frame, and T_FEC indicating the end of the frame. Referring to FIG. 4, S_FEC is equal to /K28.5/D/K28.5/D/S, in which /K28.51D/K28.5/D/ is added to /S/, which is a Start Packet Delineator (SPD) indicating the beginning of a PCS frame. T_FEC has a /T/R/I/T/R or /T/R/R/I/T/R, in which /I/T/R/ is added to /T/R/ or /T/R/R/, which is an End Packet Delineator (EPD) indicating the end of a PCS frame.
Both /K28.5/D/K28.5/D/ and /I/ indicate an IDLE code-group, and are acknowledged as idle by the 8B10B encoder and decoder. As shown in FIG. 3B, a receiver identifies a boundary of an 8B10B frame using a synchronous block of an input 8B10B code-group received from a PMA sublayer using TBI, partitions Reed-Solomon code payload and parity data, 8B10B-decodes the Reed-Solomon code payload and parity data, and converts the decoded data into byte units. The identifiers used to generate the FEC frame are removed. Converted data having byte units are Reed-Solomon <239, 255, 8>-code decoded for correcting errors, and are transmitted to a PCS sublayer using the TBI after 8B10B encoding.
A parity byte is replaced with an IDLE code, which is decoded to a zero byte through the 8B10B decoder. When a receiver receives a frame that is not FEC encoded, the receiver detects a frame with a non-FEC packet boundary detect circuit, and transmits the frame to a PCS sublayer through a matching delay. A COMMA_DETECT signal input from a PMA sublayer results from the detection of COMMA from bit data received from a PMA sublayer and the re-configuring of an 8B10B code-group based on the location of the detected COMMA. The COMMA detect signal indicates a location of the COMMA to a FEC sublayer. The COMMA is k28.5, a special code in an 8B10B code-group which makes it possible to detect the beginning of a frame.
FEC adds a 16 byte parity, and identifiers such as S_FEC and T_FEC to blocks having 239 byte units in order to generate a FEC frame. Therefore, space should be made in FEC frame to insert the additional information, which is performed in MAC. Thus, a MAC stretches an Inter-Packet Gap (IPG) according to a frame size of transmitted data in order to add parity and identifiers.
Reed-Solomon <239, 255, 8> code used in a FEC sublayer adds 16 bytes of parity to 239 bytes of information data to generate a 255-byte code word, and corrects an error of the maximum 8 bytes in the 255-byte code word including the parity. Since Reed-Solomon <239, 255, 8> code includes 239 bytes of information data, a FEC sublayer divides an input data frame received from a PCS sublayer into 239-byte blocks in order to process a frame having a variable data length with a minimum length of 64 bytes and a maximum length of 1500 bytes such as an Ethernet frame. When a last block has less than 239 bytes, the last block is zero-padded.
The following three problems occur in FEC in an EPON system. First, an OLT has to use FEC when no ONU uses FEC, and an ONU has to use FEC when the ONU has initially used FEC. As a result, FEC overhead resulting from the added FEC identifiers and Reed-Solomon code parity, throughput is reduced 35% in data transmission over the Internet. Second, FEC of an ONU is not used when there are few ONUs and an optical link is good in an initial racing stage of EPON. Later when it is necessary to use FEC due to additional ONUs, a manager or a network provider must operate existing FECs of ONUs manually. Third, when an optical link is deteriorated while not using FEC, commencement of FEC causes a change in the RTT and an ONU has to go through discovery. Therefore, a method of controlling OLT and ONUs to monitor states of an optical link automatically and determine whether to use FEC is required.
Examples of conventional auto-negotiation methods used for Ethernet will now be described. For example, in U.S. Pat. No. 6,457,055 B1, entitled “Configuring Ethernet Devices”, there is provided a method of allowing a linked device to acknowledge that a corresponding device supports a specified speed and in a user-specified transmission mode through auto-negotiation and to perform a data transmission at a user specified speed and transmission mode when a corresponding device is operated at a specific speed and transmission mode for a special reason such as a test or a limitation in design.
In U.S. Pat. No. 6,538,994 B1, entitled “Monitoring of Connection between an Ethernet HUB and an End Station”, in an initial matching between an Ethernet HUB that supports a data transmission speed of 10 Mbps and 100 Mbps and an Ethernet device that supports an optional speed of 10 Mbps and 100 Mbps, when the HUB first transmits data at 100 Mbps and measures a code error of data received from a linked device, if the measured error exceeds a designated threshold value, the HUB determines that a linked device can not support a transmission speed of 100 Mbps but only 10 Mbps and lowers a transmission speed to 10 Mbps.
In U.S. Pat. No. 5,809,249, entitled “System Having at Least One Auto-Negotiation Enabled Physical Media Dependent (PMD) Interface Device Operable to Perform Auto-Negotiation with Remote Link Partner on Behalf of All PMD”, in an initial matching between a device having a plurality of different transmission physical layers and a linked device, besides a plurality of different transmission physical layers, an auto-negotiation enabled physical media dependent (NWAY PMD) functions as an auto-negotiation master, matches each transmission physical layer and a linked device, and transmits data using a transmission physical layer having a highest matching performance.
In U.S. Pat. No. 5,922,052 entitled “Fast Ethernet Combination Chaining of Auto-Negotiations for Multiple Physical Layer Capability”, in an initial matching between a device having a plurality of different transmission physical layers and a linked device, without the assistance of an auto-negotiation physical layer, a device having a plurality of different transmission physical layers performs auto-negotiation with a linked device using a first transmission physical layer for matching. When matched, the corresponding transmission physical layer is used. When not matched, a next transmission physical layer is used to perform auto-negotiation again.
However, it is difficult to apply a conventional auto-negotiation method to an EPON system using a TDM transmission mode. As described above, an EPON system using a TDM transmission mode has to maintain RTT uniformly, and control a FEC operation according to a state of an optical link. Use of auto-negotiation in an EPON system requires a separate auto-negotiation channel, which cannot prevent a change in RTT and provide a method of measuring optical link quality.